1. Field of the Invention
This invention relates to well logging methods for determining the porosity of earth formations surrounding a borehole. More particularly, the invention relates to a method for obtaining porosity determinations in a formation of any lithology from a neutron-neutron logging tool, wherein the porosity determinations are substantially free of salinity and capture cross section effects.
2. Description of the Prior Art
Neutron-neutron logging tools having a neutron source and two neutron detectors spaced at different distances from the source are well known in the art. Such tools are particularly arranged to derive a determination of the porosity of a formation (commonly referred to as the "neutron porosity"), and an example of such a tool and the manner of obtaining the neutron porosity are described in U.S. Pat. No. 3,483,376 issued to S. Locke et al. on Dec. 9, 1969.
In the neutron-neutron tool of the art, neutrons from a neutron source (typically chemical) are emitted into the surrounding formation, and the spaced detectors are arranged to count the rate of thermalized neutrons (e.g. neutrons of an energy of 0.025 eV or less) incident their surfaces. The physical mechanism at work which permits a porosity determination to be made from the count rates of the spaced detectors is the basic fact that neutrons of high energy will most quickly lose their energy when colliding with elements having a mass similar to that of the neutron as opposed to colliding with elements having masses substantially different than that of the neutron. Since the element hydrogen (H) is the only element having a mass similar to that of the high energy neutrons, and hydrogen is found to be a substantial component of hydrocarbons and water (which comprise the porosity of the formation) but not a substantial component of most minerals of the formations, neutrons will more quickly lose their energy in more porous formations having a higher hydrogen content. By comparing the rate of thermalized neutrons detected by the far detector as opposed to the near detector, an indication of how quickly the thermalization of the neutrons in the formation (accounting for borehole and other environmental effects) is occurring becomes available, and an apparent porosity determination can be made therefrom.
Because hydrogen is the primary element at work in affecting the transport of emitted neutrons, it will appreciated that the introduction of salt into the fluid in the formation pores causes the apparent porosity to change as the salt affects the amount of hydrogen in the pores. Moreover, as will be discussed hereinafter, salt affects the porosity determination by introducing chlorine which changes the capture cross section of the formation liquid. The effects of salt on porosity have been studied (although not by distinguishing between the effects of hydrogen displacement and changed capture cross section), and corrections for the same may be had by reference to e.g. Gilchrist, Jr., W. A., et al. SPE 15540 (1986).
It will be appreciated that hydrogen is not the only element at work in affecting the transport of the emitted neutrons. Indeed, it has become readily apparent that different formations having the same porosities but containing different minerals will provide different detector count ratios. Thus, different charts (as may be seen by reference to Schlumberger Log Interpretation Charts, Schlumberger Well Services, 1986) have been made available for three common minerals (dolomite, limestone, and sandstone) so that the porosity can be determined as a function of count rate ratio and mineral.
As aforementioned, another mechanism at work in the formation under investigation which affects the ratio of the thermalized neutron count rates and hence the apparent porosity reading of a neutron-neutron tool is the macroscopic capture cross section (SIGMA .SIGMA.) of the formation. The capture cross section is an indication of how well the formation elements capture neutrons and thus preent the detectors from receiving and detecting thermalized neutrons. In other words, the higher the capture cross section, the fewer the number of neutrons will be available to be counted. While it has been recognized that the capture cross section will affect the count rates of the near and far detectors in different manners thereby affecting the porosity determinations, the art has not provided a comprehensive manner in dealing with the same. Rather, the manner in which the count rates are so affected have been studied for the three aforementioned common minerals filled with freshwater, and charts relating the migration length of the neutrons (L.sub.m) (i.e. the root mean square distance the neutron travels until it is captured) with the count ratios and porosity have been provided in helping to determine the true porosity of the formation. (See e.g. H. Edmundson, et al., "Radioactive Logging Parameters or Common Minerals" SPWLA 20th Ann. Logging Symposium, June 3-6, 1979.
While the salinity, lithology, and capture cross section effects on the porosity readings of the neutron-neutron tool have been studied, it will be appreciated that in formations having lithologies which do not directly correspond to pure limestone, sandstone, and dolomite, a corrected determination of porosity is not readily available. Also, where the water in the pores of the formation has a capture cross section which is not a function of salinity only, a corrected determination of porosity is not readily available.